The present invention relates to vehicle transporters, such as trucks, trailers, and the like, that have vehicle support members movable relative to the frame of the transport vehicle and, more particularly, to a vehicle transporter having a vehicle support member movable by a screw actuator.
Vehicle transporters are normally equipped with elongate vehicle support members to engage and support the wheels of the vehicles comprising the cargo. The vehicle support members may be fixed to the vehicular frame of the vehicle transporter, but are often movable relative to the vehicular frame to permit orienting the cargo vehicles so that the payload can be maximized and the height of the transporter reduced to satisfy legal requirements and to clear overpasses and other obstacles. The movable vehicle support members can also be positioned to form a surface over which cargo vehicles can be driven during loading and unloading.
The movable vehicle support members are, typically, moved by means of elongate hydraulic cylinder assemblies connecting the vehicular frame and the vehicle support members. However, a significant drawback of such transporters is the time required to mechanically lock each hydraulic cylinder assembly in position when the transporter is loaded and unlock each cylinder assembly so that the associated vehicle support members can be repositioned during loading and unloading. Mechanical locking is important to maintain the position of a vehicle support member in the event that the hydraulic cylinder assembly does not continue to support the load due to a broken fluid supply line, seal failure, leakage, inadvertent control actuation, or some other reason. This task typically requires the manual insertion or removal of a pin at each of the hydraulic cylinder assemblies. Since a vehicle transporter may have 16 or more pairs of hydraulic cylinders, half of which are typically located on each side of the vehicle transporter's frame, correctly positioning the hydraulic cylinders and manually locking or unlocking each cylinder is very time consuming.
Andre et al., U.S. Pat. No. 5,938,382, disclose the use of screw drives for positioning vehicle supporting members on an over-the-road vehicle transporter. Each vehicle supporting structure is supported by at least one pair of laterally spaced screw drives. The screw drives comprise a powered screw supported in tension from its upper end in a substantially vertical hollow post. Each screw is rotated by a hydraulic motor having a shaft attached to the bottom of the screw and a case attached to the transporter's frame. A nut, that is captive in the post, is displaced along the screw when the screw is rotated by a motor attached to the lower end of the screw. The cross-section of the post is a C-shaped channel and a portion of the nut projects through the gap in the channel section and is attached to a vehicle supporting member. The posts are fixed and, typically, substantially vertical to avoid side loads that might bend the screw or damage the motor bearings. The fixed, vertical posts complicate the connections to the vehicle supporting members which are often pivoted about one end to facilitate orienting the cargo vehicles to maximize the number carried by the transporter. In addition, the gap in the channel-shaped cross-section of the post exposes the screw and nut to the elements, including moisture and road salt, in the harsh over-the road environment.
To synchronize rotation of motors powering a pair of laterally spaced screws and, therefore, the translation of the movable nuts supporting a vehicle support structure, the motors are hydraulically connected in series so that the exhaust of the first motor is the supply for the second motor. Each motor is connectable to the reservoir and to the pump supplying pressurized fluid. Each motor is also connected to its paired motor by a fluid line extending across the transporter's frame. In addition, the supply and exhaust ports of each motor of the pair must be cross connected, through a pair of relief valves, to the ports of other motor so that leakage does not prevent one of the actuators from moving through the full range of motion. While a series fluid connection roughly synchronizes the operation of a pair of fluid actuators, each actuator must exhaust exactly the volume that is required to supply the other actuator or some circuitry must be provided to account for the difference increasing the number of valves, supply lines, and connections in the fluid supply and control system.
In the alternative, the paired hydraulic actuators can be connected in parallel. However, the movement of hydraulic actuators connected in parallel is not synchronized and the actuator experiencing the lowest pressure will move first and fastest. If the actuators are connected in parallel, a means must be provided to equalize the displacement of the actuators because differences in the internal construction of the actuators, friction or binding in the connections for the vehicle supporting structure, or side-to-side differences in the weight of the cargo vehicle commonly causes unequal displacement of the laterally spaced actuators of a pair of actuators supporting a vehicle supporting structure.
What is desired, therefore, is a self-locking actuator that is well protected from the environment and conveniently connectable to the various movable and stationary members of the structure of a vehicle transporter.